1. Technical Field
This invention relates to a process for treating septage whereby septage is defined as liquids, solids, and semi-solid contents of privies, chemical toilets, cesspools, septic tanks, holding tanks, dry pits, grease traps, grit traps, boat pump out stations, or other sewage waste receptacles. More specifically, the invention involves a process for removing pathogens from the septage and for controlling vector attraction while producing bio-solids that can be utilized as a fertilizer or other beneficial use such as soil conditioner. Additionally, the process allows for the removal of the majority of the water component of the septage and provides for its treatment and subsequent discharge into the environment.
2. Background of the Related Art
The proliferation of fast food restaurants and other large scale food processing centers, where food preparation inherently produces fats and grease waste, in conjunction with increasing regulations concerning the disposal of such wastes, have produced a need for an efficient process for its removal and processing in order to control cost and minimize harm to the environment.
Food, grease and oil are present in waste water produced from restaurants and other food processing establishments. This waste water containing fats, grease and oils forms an emulsion that passes through a grease trap, which is required by most local and state regulations, to capture components within the waste water preventing their release into municipal sewer systems or septic systems. Typically, the grease trap comprises an infall pipe, which discharges the water containing grease and food into an underground holding tank capable of retaining large quantities of this unwanted waste. These holding tanks contain barriers that trap and partially separate heavy solids, floating trash and much of the fats, oils and grease produced from food processing and preparation area drains. As a matter of design these grease traps become saturated with solids, trash, fats, oils and grease, and must be drained periodically per local and state health regulations. The draining of these grease traps is accomplished by a tanker truck having a vacuum pump that retrieves the contents of the grease trap for proper disposal.
The discharge of this concentrated wastewater containing solids, trash, fats, oils and grease into a conventional municipal wastewater treatment facility will either deteriorate the efficiency of that municipal facility or cause serve operational difficulties within the municipal wastewater system. In light of these difficulties, there is a reluctance of municipally owned wastewater treatment plants to accept septage and more particularly grease-trap wastes.
The evolution of greater regulatory requirements as to the disposal of these wastes has contributed to escalating costs associated with their proper disposal. Historically, this septage was disposed of by dumping or landfilling the waste after the removal of some of its aqueous components. Unfortunately, untreated septage, both in solid and liquid form, may contain any number of substances toxic to humans and the environment, including, solvents, organic and inorganic compounds and pathogens. The treatment of septage to destroy these pathogens and noxious compounds before placement within a landfill has become increasingly more important due to environmental concerns. Greater interests in the environmental impact of the disposal of septage and the resulting increase in regulatory requirements have caused escalating costs associated with the proper disposal of septage.
Different methods of treating septage have been implemented with varying degrees of success. One prior art method was to dump the septage into an open pit, where it was then mixed with ash and dirt forming a substantially dry material. This resulting dry material was then placed within a landfill. However, this method required valuable and costly landfill space and also had significant odor problems. Additionally, this virtually unprocessed method of treating septage has certain environmental concerns along with vector attraction.
Many newer approaches to septage treatment utilize labor, energy, intensive chemical and biological systems in order to process the septage so that it is suitable for discharge within the environment. Unfortunately, such processes are costly due to the cost of labor, energy and the expense associated with biological and chemical processes. Moreover, these processes often produce materials that are large in volume and have no practical use. Additionally, these chemical and biological treatments of septage can still have detrimental effects on the environment.
One of these methods utilizing chemical and biological treatment uses surfactants to break down fat globules contained within grease trap waste. Once the fats are broken down, microbes are used to ingest the fat particles. Unfortunately, this process is extremely cost prohibitive and labor intensive and has been met with limited success. Additionally, this method also suffers from significant odor problems and vector attraction. Most importantly, this method still requires a significant volume of landfill space.
Another method involves the heating of the waste to remove by skimming the fats contained within such waste. The solids within the waste are removed and the subsequent liquid waste is heated again to remove remaining pathogens. Unfortunately, this process is both labor intensive and costly from an energy standpoint due to the multiple heating of the waste product. Additionally, the removal of solids within such process produces a product that has no known use and is subsequently disposed within costly landfill sites. Furthermore, the solids produced from this process have not been treated for pathogens or vector attraction.
Several recent approaches have been developed to treat the sludge generated at municipal wastewater treatment plants. These processes treat the sludge with a selected alkaline additive such as lime. One method uses the exothermic reaction between the alkaline additive and the water contained in sludge to produce sufficient heat within the sludge to destroy existing pathogens. Another method uses less lime for the exothermic reaction but supplies supplemental heat to the sludge by electrical elements to attain the temperature required to destroy existing pathogens. Both processes use the elevated pH of the sludge caused by the lime addition to reduced vector attraction. Unfortunately, the energy and chemical. costs associated with these processes are extremely high. More importantly, the heating of sludge, a solid material, by the use of electrical elements or excess lime addition does not necessarily produce uniform heating throughout the sludge and subsequently contributes to pathogen reduction that is not predictable.
As a result of the deficiencies of prior art treatment methods in addressing landfill usage, energy costs and detrimental effects on the environmental, there is a need to provide a septage treatment process that has a low, if not beneficial, impact on the environment while being cost effective.
The present invention is a waste processing facility specifically designed to process septage. Septage is the liquid, solid, and semi-solid contents of privies, chemical toilets, cesspools, septic tanks, holding tanks, dry pits, grit traps, boat pump out stations, grease traps or other sewage waste receptacles. The physical characteristics of septage vary widely. Septage has a percent solids concentration of generally 0 to 10% by volume. The processing facility is designed to handle approximately 100,000 gallons per day of septage during a 12-hour shift, although other varying amounts are contemplated herein. The inventive process will convert the septage to approximately 40 tons of Biosolids (sludge) having a composition of approximately 60% water. The inventive process addresses the following two considerations in the treatment of septage: pathogen reduction (e.g., helminthes (intestinal worms), bacteria, protozoa, and viruses, etc.), and vector attraction (e.g., rodents, birds, insects, and other organisms that can transport pathogens).
According to the inventive method, the septage is treated upon arrival at the processing facility by screening and grinding the raw septage. After the initial treatment, the septage is stored within a receiving tank prior to being transferred to a pasteurization tank. The septage is transferred, in batches, to pasteurization tanks and is pasteurized by the introduction of steam into the tanks thereby elevating the temperature of the septage to 70 degrees Celsius (158 degrees Fahrenheit) or higher for a minimum of 30 continuous minutes. Due to the more efficient thermal conductivity of water compared to solids, heating the septage, with its much higher water content, is more effective at maintaining even temperatures throughout the mass of material than prior art methods which heat processed sludge. Thus, heating (i.e., pasteurizing) the liquid septage as opposed to the higher solids content sludge allows for a more uniform and thorough heat treatment and ensures greater pathogen reduction. Due to the ease of handling a liquid versus a solid and the use of direct heating versus indirect heating, the cost of pasteurization is much lower.
After pasteurization, the septage is transferred to cooling tanks. The cooled pasteurized septage is then transferred to filter press feed tanks where it is treated with an alkali substance such as lime. Sufficient alkali, i.e., lime, is added to the septage to raise the pH to at least 12 and without the addition of more alkali maintain a pH in the filter cake (i.e., sludge) of at least 12 for two hours and a pH of 11.5 for an additional 22 hours. The current federal regulations related to reducing vector attraction requires that the pH of the sludge be maintained at least 12 for two hours without the addition of more alkali and thereafter that it be maintained at a pH of 11.5 for an additional 22 hours without the addition of more alkali. It is contemplated herein that other pH levels and time parameters may be used.
The process of adding an alkali such as lime also binds up the grease and other difficult to dewater materials in the septage. The added alkali acts as a filter aid and allows the septage to be more easily dewatered into a good filter cake. The resulting lime conditioned septage slurry is pumped through filter presses. The filter presses separate the solids from the liquid. The lime that has been added during the process is mostly non-soluble and is retained within the sludge.
The sludge which is dropped from the filter presses has undergone treatment and meets federal and state standards for beneficial reuse as a Biosolid. The Biosolids can be sold or given away to provide nutrients to growing vegetation or to improve the quality of soil for the purpose of growing vegetation.
The liquid that passes through the filter presses flows into an equalization tank that acts as a surge tank. The liquid is then pumped into a neutralization/precipitation tank. Due to the alkali addition, this liquid has a very high pH. In order to neutralize the liquid prior to the biological treatment processes, carbon dioxide is dissolved into the liquid to form carbonic acid. The dissolution of carbon dioxide and the resulting formation of carbonic acid and other species (e.g., bicarbonate and carbonate) lowers the pH. If lime is used as the alkali, calcium carbonate (limestone) precipitates out of solution as the pH of the liquid is lowered. The calcium carbonate drops to the bottom of the neutralizing tank and is pumped to the filter press feed tanks to be removed by the filter presses and added to the sludge.
Once the calcium carbonate is removed, the remaining liquid flows into equalization tanks. The liquid within these equalization tanks has a pH of between 7 and 8.5. The liquid is then pumped into tanks for biological treatment which includes both aerobic and anoxic processes. In the first of the aerobic processes, aerobic organisms consume the carbon (organic material) contained within the liquid thereby reducing its Biological Oxygen Demand (xe2x80x9cBODxe2x80x9d). BOD is a measure of the amount of oxygen that is used by the organisms while consuming the carbon. The second aerobic biological treatment process converts ammonia nitrogen (NH3xe2x80x94N) into nitrate nitrogen (NO3xe2x80x94N). In the anoxic process, anaerobic organisms convert the nitrate nitrogen into nitrogen gas that is dissipated into the atmosphere. The water that remains after the biological treatment steps is pumped into clarifiers. In the clarifiers, biomass which sloughs off the biological processes is removed from the water. The clarified water is then pumped through a sand filter to remove suspended solids from the water. The filtered water is then disinfected by the use of Ultra Violet (UV) filtration which kills any potentially live organisms which may have passed through the clarifiers and sand filter. The disinfected water is then discharged to the groundwater, surface water or to a sewer.
Features of the invention include but are not limited to the following: the inventive method pasteurizes septage as opposed to the pasteurization of sludge which is known in the industry and the inventive method creates a usable material (BioSolid) from septage.